(1) Field of the Invention
The invention is related to a layer application device for an electrostatic layer application of a building material in powder form and in particular to a device and a method for a layer-wise manufacturing of a three-dimensional object, which device and method apply the layer application device.
(2) Description of Related Art
A device and a method for the manufacturing of a three-dimensional object by selective laser sintering are for example known from U.S. Pat. No. 5,908,569. A respective device is shown in FIG. 1 and will be briefly described in the following.
The laser sintering device shown in FIG. 1 has a container 1 that is open to the top, in which a support 4 for supporting the object 6 to be formed is provided. By means of a drive in the container the support 4 can be moved, up and down in a vertical direction. Above the container 1a laser 14 is arranged as irradiation device that emits a directed light beam 15. Via a deflection device 16 this light beam 15 is deflected towards the object 6 to be manufactured layer-wise.
Furthermore, an application device 12 for applying a powder material 7 that is to be solidified onto the support surface 5 or a previously solidified layer is provided. In the prior art various application devices are known, by which the powder is mechanically applied and smoothed. As an example FIG. 2 shows an application device from WO 2006/122645, which is a coating device 24 having two blades 21a and 21b, which are held together at their sides by two sidewalls in parallel to the drawing plane, which are not shown. The two blades and the two sidewalls together form a supply chamber 22 that is open to the top and to the bottom and serves for receiving the powder material for a layer. By moving the coating device 24 into the direction of the arrow B the powder 27 from the supply chamber 22 is spread across the surface 5, wherein the powder layer is smoothed down by the blades 21a and 21b. 
In the laid-open German Patent Application Document DE 199 52 998 a device for a direct manufacturing of bodies is described, in which two building chambers and two powder supply chambers, respectively, lie opposite to one another similarly to quarters of a circle. The powder application in the building chambers there occurs in such a way that a cover plate of semi-circular shape, which respectively covers to adjacent chambers, is rotated. By the straight edge of the cover plate, which is formed to be a doctor, the powder is then shifted from a supply container to the adjacent building container.
In DE 10 2004 022 385 A1 the powder application is also effected by means of a rotatable doctor that is designed such that it has the shape of a ring, wherein the ring as a whole is rotated around an axis of rotation that lies outside of the ring.
Also U.S. Pat. No. 4,863,538 discloses a method and a device for manufacturing objects by means of selective sintering. In the document a pre-defined amount of the powder material is put on a support that can be lowered and is spread by means of a roller that can be moved across the support while at the same time being rotated in a direction opposite to the direction of movement across the support.
In all methods of the prior art a blade, a roller or a doctor is used for the layer application. However, the blade, the doctor or the roller in all cases applies mechanical force not only to the powder. When the powder layer has been applied onto a previously solidified layer of the part to be manufactured, then inevitably portions of the part that are already present are subjected to this force, which in general acts in a shearing way. From this a crack or a deformation may result, in particular in fragile structures or thin-wall structures. This has disadvantageous effects on the aspect ratio and the dimensional accuracy of the object to be formed.